The Consequences of Ant-Attendance to the Biological Control of the Red Wax Scale Insect Ceroplastes rubens by Anicetus beneficus

1. The effects of an attendant ant (Lasius niger) on the host-parasitoid system of the red wax scale insect (Ceroplastes rubens) and its parasitoid wasp (Anicetus beneficus) and in relation to biological control efficiency of the wasp were examined by behavioural field observations, ant exclusion experiments in the field, and field censuses of the host population density and the percentage of parasitism. 2. Parasitoids were frequently observed to interrupt their ovipositional behaviour due to interactions with ants attending host aggregations. 3. Ant-exclusion experiments revealed that ant-attendance caused a decrease in the percentage parasitism and consequently an increase in the growth rate of the host population. 4. The results of field censuses showed that, under the natural conditions in which some generalist ant species usually attended the host aggregations, host density remained at a high level or gradually increased over a 5-year period. Inverse density dependence was detected in the parasitism, which was expected from the density-biased defensive effects of ant-attendance. 5. Overall, these results indicate that ant-attendance on C. rubens restricts the ovipositional ability of the parasitoid and thus reduces the effectiveness of the parasitoid as a control agent of C. rubens.